1. Field of the Invention
The present invention relates to a sheet processing apparatus that processes a sheet and an image forming apparatus provided therewith.
2. Description of the Related Art
Conventionally, for an image forming apparatus that forms an image in a sheet, there is well known an image forming apparatus provided with a sheet processing apparatus. The sheet processing apparatus is configured such that a folded booklet is formed by stapling of a bundle of sheets in which images are formed by the image forming apparatus. Hereinafter, the booklet is referred to as saddle-stitch bookbinding, and an apparatus that performs a saddle-stitch bookbinding process is referred to as a saddle-stitch bookbinding apparatus. The sheet processing apparatus sequentially receives the sheets in a tray, aligns the sheets with one another in the form of bundle, and performs stapling near the center in a conveying direction. A projecting member strikes the stapled part to push the stapled part into a nip of a pair of folding rollers, and the sheet bundle is folded while conveyed by the pair of folding rollers. (see U.S. Patent Application Publication No. 2007/060459 A1)
Action of the conventional sheet processing apparatus will be described with reference to FIGS. 2, 16, 17 and 18. In FIG. 2, only part necessary for the conventional technique is extracted and described.
As illustrated in FIG. 2, in a finisher (sheet processing apparatus) 500, a sheet stopper 805 sequentially receives and aligns plural sheets conveyed to an accommodation guide 803, and a stapler 820 staples a central part in the conveying direction. Then, a projecting member 830 strikes the stapled part of the sheet bundle to push the stapled part into a nip of a pair of first folding rollers 810a and 810b. The pair of first folding rollers 810a and 810b folds the sheet bundle in two while conveying the sheet bundle, and sequentially conveys the sheet bundle to a pair of second folding rollers 811a and 811b and a pair of third folding rollers 812a and 812b. 
The sheet conveyance is tentatively stopped when a folding end part of the folded sheet bundle is conveyed a process position of a pair of press rollers 861. The pair of press rollers 861 moves along a fold line of the sheet in a width direction orthogonal to the sheet conveying direction to perform strengthening process to the folded part (see FIG. 16). Then, the folded sheet bundle is conveyed and discharged to a folded bundle tray 840.
In the conventional finisher, both the sheet stapling process and the striking and folding process are performed in the center of the sheet. That is, in stacking the sheets, the stapler 820 performs the stapling process to sheets P after the sheets P are sequentially stacked on the sheet stopper 805 that waits below by a half of a length L in the conveying direction of the sheet bundle P around the stapling position of the stapler 820, that is, L/2.
Then, both the sheet bundle P and the sheet stopper 805 move by a distance L1 from the center of the stapler 820 to the nip center of the pair of first folding rollers 810 such that the nip center of the pair of first folding rollers 810 and the stapled part of the sheet P are matched with each other (see FIG. 16).
For example, for the reason an apparatus width is decreased, sometimes the pair of first folding rollers 810, the pair of second folding rollers 811, and the pair of third folding rollers 812 are not linearly disposed, but configured as a curved conveying path as illustrated in FIGS. 2 and 16. That is, the pair of second folding rollers 811 is located slightly above the pair of first folding rollers 810, the pair of third folding rollers 812 is located below the pairs of first and second folding rollers 810 and 811, and the nip direction of the pair of second folding rollers 811 is oriented downward to the left. Totally, in FIG. 16, the nip of the pair of press rollers 861 is located below the nip of the pair of first folding rollers 810.
However, the following state is generated when the sheet bundle P in which the stapling process and the striking and folding process are performed to the central part thereof is conveyed through the bent conveying path. That is, the leading end of the sheet bundle is oriented downward by an upper guide 814a in the upper guide 814a and a lower guide 814b of a conveying guide portion 814 between the pair of folding rollers 811 and 812. Therefore, in the sheet bundle P, an outside (upper side) stretches by a conveying resistance from the upper guide 814a while an inside (lower side) sags.
When the stretch state and the sag state are generated in the sheet bundle P, a staple S performed in parallel to the sheet bundle P is oriented upward.
Then, the sheet bundle P is conveyed to the pair of third folding rollers 812, the sheet bundle P is conveyed to the pair of press rollers 861 and a pair of crushing rollers 862 while the staple S is oriented upward as illustrated in FIG. 17, and the conveyance of the sheet bundle P is tentatively stopped.
The sheet bundle P is conveyed to the pair of press rollers 861 and the pair of crushing rollers 862 while deviated in a front-back direction of the apparatus (a front-depth direction of FIG. 2), and the conveyance of the sheet bundle P is tentatively stopped. Then, the pair of press rollers 861 and the pair of crushing rollers 862 move in a direction of an arrow a of FIG. 18, and an press process and a crushing process are performed to the leading end of the folded part near the staple S of the sheet bundle P. The pair of additionally folding rollers 861 applies a force in a direction of an arrow Fp of FIG. 17 and the pair of additionally folding rollers 861 can swing in a direction of an arrow Rp of FIG. 17. The pair of crushing rollers 862 applies a force in a direction of an arrow t of FIG. 17, and can swing in a direction of an arrow Rt.
In the conventional finisher, because the press process and the crushing process are performed while the staple S is oriented upward, the staple S is oriented upward in the final deliverable, and the quality of the saddle-stitch sheet bundle P is finally degraded. That is, the sheet bundle is conveyed through the curved conveying path to deform the leading end of the folded part of the sheet bundle, and the press process and the crushing process are performed while the staple S is oriented upward. Therefore, the staple S is deviated to generate the final deliverable in which the quality is degraded.
The present invention provides a sheet processing apparatus that can solve the problems such as the reduction of productivity, the cost increase and degradation of accuracy of sheet position control, and an image forming apparatus provided with the sheet processing apparatus.